Some available document is absolutely required, preferably through a link. If I don't see that, then your thread gets bumped to the Inactive File, do not pass Go, et cetera.

I was wondering what sort of document is required.Does it need to be a pdf? Word document?

Would it be alright if I just start some sort of blog to contain my random musings and brainstorming, and link back to it when I start a thread on these forums asking a specific question in regards to game design?

Any kind of document at all. Format doesn't matter as long as it's something most or many people can access.

A blog is fine, but it should include something more than merely random musings. If nothing else, some kind of explicit organizing or titling system which allows people to find the rules in their current state. Most blogs allow you to tag posts with identifying terms, so that should be pretty easy.

Thanks.I'm still working on my first draft. Would I need to post my completed second draft before I can start asking for feedback on it?Or can I post my second draft in installments and get feedback as I go?

I can't monitor the versions you're working on, so keep the available ones as up to date as you can, without going crazy. In other words, upload your first draft, then start posting whatever thread you'd like, and keep going from there.